I wear one earbud (the right one) and listen to either music or audiobooks at moderate volume - loud enough to hear over the wind noise (my Skullcandy IEMs block most of that, anyway) but not so loud as to make my right ear useless. If I can hold a conversation with another rider at a stoplight without having to pause my music, I figure I'm fine, and it significantly increases my enjoyment of commuting.

I usually tuck my left earbud into my jersey so it's not dangling, and the right one has its cord under the helmet strap so if it comes out (or, occasionally, I want to hear more of my surroundings and take it out) it doesn't fall down - I don't need to get my earbuds tangled in my spokes.